The present invention relates to a process for leaching zinc containing materials which are usually, but not necessarily, minerals in order to recover metal values contained therein with the object of providing a hydro-metallurgical process for treatment of such sulphide containing materials.
The processing of metal sulphide containing minerals or ores is attractive from a number of points of view. Firstly, the sulphide minerals can be concentrated by conventional beneficiation techniques, such as, for example, suitable flotation techniques. Secondly, there is, as a result of the hydrometallurgical nature of a leaching process carried out directly on the sulphide containing minerals, no air pollution problem which generally necessitates the construction of a costly sulphuric acid plant for converting the sulphur dioxide formed into sulphuric acid, and thus by using such a hydrometallurgical technique capital expenditures can be reduced. Thirdly, when a sulphide mineral is roasted, in accordance with existing practice, the metals therein are converted to oxides which, after leaching of the majority of the metals contained in such oxide roast product, cannot be easily treated for recovery of the remaining oxides which are unleached. Thus, the residual unleached metal oxides are often discarded as a waste product together with the leach residue.
Zinc sulphide containing minerals have, in the past, been treated by various leaching processes. One of these which involves the direct leaching of a finely ground mineral or ore by sulphuric acid is described in U.S. Pat. No. 2,996,440 to Forward et al. In that process, as is usual, the sulphuric acid is the spent electrolyte from the subsequent electrowinning step. This single stage leaching process proved to be uneconomically slow at the low pressures and temperatures. Since elemental sulphur is liberated in this process the temperature should be maintained below the melting point of sulphur viz. about 119.degree. C. and thus the kinetics cannot be improved by simply raising the temperature.
Subsequent work on this single stage leaching process led to an improvement which is described in British Pat. No. 1,000,121 wherein iron was added to the leach slurry in amounts of about 5 to 15% by weight of the non-ferrous metals being leached. This improved the kinetics of the process which, as will be clear from the above, involved the direct leaching of the mineral or ore with the spent electrolyte. However, the kinetics were, applicant believes, still not satisfactory as a result of which high pressure, high temperature leaching processes were investigated.
These investigations led to various processes being developed and patented in which such mineral sulphides in finely divided form, are pressure leached in an aqueous sulphuric acid solution under oxidizing conditions. Such leaching is generally effected at temperatures above about 135.degree. C. but below about 175.degree. C. A two-stage pressure leaching process of this nature forms the subject matter of U.S. Pat. Nos. 4,004,991 and 4,071,421. In the patented process the sulphur present in the sulphide material again converts to its elemental form and a zinc sulphate bearing leach solution is produced which, after purification, is subjected to electrolysis to recover cathode zinc and to re-generate the aqueous sulphuric acid which is recycled to the leach step. However, leaching at such elevated temperatures results in significant disadvantages including the following:
(i) The process utilises air, or preferably oxygen, to create the oxidising conditions and the partial pressure of the oxygen present must be sufficient to provide a reasonably satisfactory rate of reaction. Since the vapour pressure of water at 135.degree. C. is about 300 kPa and increases up to about 900 kPa at 175.degree. C. high pressures must be utilised in order to provide a satisfactory oxygen partial pressure. This, of course, necessitates the construction of equipment capable of withstanding such high pressures at the elevated temperatures which promote vigorous chemical reactions.
(ii) The use of air to provide the oxygen necessitates that a large gas bleed from the system be provided in order to remove the inert nitrogen and other gases. As a result there will be a high loss of water, and therefore heat associated with such water. Also, there is required capital expenditure to provide suitable condensation equipment to recover the evaporated water.
(iii) Leaching with aqueous sulphuric acid (having an initial acid concentration typically of about 180.0 g/l) at such elevated temperatures, requires that special, and therefore costly, materials of construction must be used for the pressure leaching apparatus.
(iv) Safety hazards associated with the operation of a process under elevated temperature and pressure as indicated above are greater than for a process operating under milder conditions.
(v) As the leaching temperatures under consideration fall above the melting point of elemental sulphur, surface active reagents need to be added to prevent molten sulphur from blinding the mineral surface and thereby inhibiting the leaching. The use of such reagents can add significantly to the cost of the process.
It is accordingly the object of this invention to provide a process for the treatment of zinc sulphide containing materials which involves leaching under less vigorous conditions than are present in the aforementioned high temperature, high pressure process, but nevertheless has acceptable kinetics for industrial application.
It is therefore an object of this invention to enable less costly equipment to be utilised for conducting a leaching process on zinc sulphide containing materials.